Typically, electric linear motion actuators for converting rotary motion of an electric motor to linear motion, thereby linearly driving a driven member use a ball-screw mechanism or a ball-ramp mechanism. Also, in order to produce a large linear driving force with a small-capacity electric motor, a gear reduction mechanism such as a planetary gear reduction mechanism is often used (see e.g. JP Patent Publication 6-327190A).
While ball-screw mechanisms or ball-ramp mechanisms as used in electric linear motion actuators can amplify force to a certain extent with its motion converter mechanism due to threads having a lead angle or inclined cam surfaces. But these mechanisms cannot amplify force to such an extent as to be expected for e.g. electric brake assemblies. Thus, electric linear motion actuators including such a motion converter mechanism further include a separate speed reduction mechanism such as a planetary gear reduction mechanism to amplify driving force. By mounting such a separate speed reduction mechanism, it becomes difficult compactly design the electric linear motion actuator.
In order to avoid this problem, the present inventors proposed an electric linear motion actuator which can sufficiently amplify force without the need for a separate speed reduction mechanism and which is suitable for use in an electric brake assembly, of which the linear motion stroke is short. This linear motion actuator comprises a rotary shaft to which rotation of a rotor shaft of an electric motor is configured to be transmitted, a casing, an outer ring member fixed to a radially inner surface of the casing, a carrier, and a plurality of planetary rollers rotatably supported by the carrier and disposed radially outwardly of the rotary member between the outer ring member and the rotary member such that the respective planetary rollers revolve around the rotary shaft while rotating about their respective axes as the rotary shaft rotates, wherein a helical rib is provided on a radially outer surface of the rotary shaft or a radially inner surface of the outer ring member, and wherein each of the planetary rollers has on its radially outer surface circumferential grooves which are identical in pitch to the helical rib and in which the helical rib is engaged, or a helical groove which is identical in pitch to and differs in lead angle from the helical rib and in which the helical rib is engaged, whereby the carrier is axially moved relative to the rotary shaft as the rotary shaft rotates and the planetary rollers revolve around the rotary shaft while rotating about their respective axes, thereby converting rotary motion of the rotary shaft to linear motion of the carrier (see JP Patent Publication 2007-32717A and JP Patent Publication 2007-37305A).
On the other hand, while conventional vehicle brake assemblies are mostly hydraulic ones, with the introduction of sophisticated brake control such as ABS (anti-lock brake system), electric brake assemblies are gathering attention, because electric brake assemblies can perform such sophisticated brake control without the need for complicated hydraulic circuits. Electric brake assemblies actuate an electric motor based on e.g. signals indicative of depression of the brake pedal, thereby pressing the brake member against the member to be braked through the above-described electric linear motion actuator, which is mounted in the caliper body (see JP Patent Publication 2003-343620A). Since such an electric brake assembly is mounted under a vehicle spring, it is desired that such a brake assembly be compact in size and operate stably under vibrations transmitted from the road surface.
The electric linear motion actuators disclosed in JP Patent Publication 2007-32717A and JP Patent Publication 2007-37305A are compact in size because no separate speed reduction mechanism is mounted and still can produce large force. But because a preload is applied to the planetary rollers by fitting the planetary rollers between the radially outer surface of the rotary shaft and the radially inner surface of the outer ring member with a negative gap, it is necessary to mount the planetary rollers between the outer ring member and the rotary shaft by e.g. shrink fitting. Also, in order to control the negative gap for the preload, it is necessary to finish the radially outer surface of the rotary shaft and the radially inner surface of the outer ring member to high dimensional accuracy by e.g. grinding. Thus, additional time is necessary for the steps of finishing the rotary shaft and the outer ring member and of mounting the planetary rollers, which pushes up the manufacturing cost.